The electron microscope is an indispensable component of a laboratory engaged in the study of gastroenteritis viruses. Two major groups of gastroenteritis viruses -- the 27nm Norwalk virus and the 70nm human rotaviruses -- were discovered at NIH and in Australia, respectively, (as well as the hepatitis V virus at NIH, in collaboration with the Hepatitis Section) by the use of electron microscope techniques. These agents were discovered without the use of an in vitro tissue culture system, because they could not be grown from clinical specimens in cell culture. The term, "direct virology", is apt to describe this method of examining viruses from clinical specimens by electron microscopy. Although second and third generation tests have been developed for the detection of the Norwalk group of viruses and the rotaviruses, the electron microscope is still an indispensable tool for the study of these gastroenteritis viruses. It is also the most rapid diagnostic method for detection of rotavirus from a clinical specimen and is the only method available for detecting infection with certain fastidious 27nm or similar small round virus-like particles from individuals with viruses associated with epidemic nonbacterial gastroenteritis. In addition, it is the only method that is capable of detecting all known gastroenteritis viruses (e.g., group A or non-group A rotavirus, Norwalk-like viruses, adenoviruses, astroviruses, caliciviruses) by examination of a single stool specimen. It has also been employed recently in collaborative studies for the visualization of baculovirus expressed Norwalk virus outer capsid antigen and a baculovirus expressed hepatitis E antigen.